Any status you earn between now and Dec 31 is for 2013. So if you get SM now you will be SM until Feb 2014.
The Amex will give you a nice boost, priority boarding, etc. if you plan on only flying Delta do it. You won't regret it. There are a lot of Medallions in ATL and it isn't uncommon for Zone 3-4 to have to gate check bags.
If you will travel weekly, consider the DL Reserve card, which also gets you into the SkyClubs.